1. Field of the Invention
The present invention relates to an overvoltage protection circuit which protects a circuit from an overvoltage.
2. Description of the Related Art
A circuit element used in a semiconductor integrated circuit is not able to execute a normal function when applied with a voltage exceeding a withstanding voltage. In electronic equipments that operate using an external power supply, in particular, an equipment presumed to be employing an emergency power supply using a dry battery or a USB (Universal Serial Bus) power supply of unsatisfactory quality, the overvoltage protection circuit which protects the circuit element from overvoltage is required since there is a possibility high voltage outside the rating will be applied.    [Patent document 1] Japanese Patent Application (Laid Open) No. 2004-70666
The overvoltage protection circuit is generally arranged between an input terminal and an output terminal, and includes a switch element which is turned OFF when the voltage of the input terminal (hereinafter referred to as input voltage) exceeds a threshold value. In the case of such configuration, the voltage of the output terminal (hereinafter referred to as output voltage) is suppressed to smaller than or equal to a threshold voltage. However, the output voltage also fluctuates if the voltage level of the external power supply fluctuates in a region lower than the threshold voltage.